Spectral Properties of Semiconductor Photodiodes

Needs for quantitative optical measurements are expanding in various applications where measurement conditions are very different. For precise measurements, uncertainties caused by difference in measurement conditions should be taken into consideration. Measurement conditions for the use of photodiodes include what kind of source is used like whether it is monochromatic or continuum spectrum, collimated or divergent, polarized or unpolarized, what the beam geometry is like whether it is oblique incident or normal incident, underfilled or overfilled, what power level the detector receives and so on. Since photodiodes are optoelectronic devices, both optical and electronic properties are important. Contrary to electronic properties of photodiodes, optical properties, especially spectral properties like polarization dependence and beam divergence dependence have seldom been reported except from the author’s group (Saito, T. et al., 1989; Saito, T. et al., 1990; Saito, T. et al., 1995; Saito, T. et al., 1996a; Saito, T. et al., 1996b; Saito, T. et al., 2000). Most photodiodes can be optically modelled by a simple layered structure consisting of a sensing semiconductor substrate covered by a thin surface layer (Saito et al., 1990). For instance, a p-n junction silicon photodiode consists of a silicon dioxide film on silicon substrate and a GaAsP Schottky photodiode consists of a gold film on GaAsP substrate. Even with a single layer, optical properties of the whole system can be very different from those for a substrate without surface layer due to the interference effect and absorption by the surface layer. To understand spectral properties like spectral responsivity and polarization responsivity dependence on angle of incidence, rigorous calculation based on Fresnel equations using complex refractive indices of the composing materials as a function of wavelength is necessary. When the incident photon beam is parallel and there is no anisotropy in the sensing surface, there is no need to consider on polarization characteristics of photodiodes. However, when incident beam has a divergence, one has to take polarization properties into account since there are components that hit detector surface at oblique incidence (Saito et al., 1996a). To measure divergent beam power precisely, detectors ideally should have cosine response. Deviation from the cosine response also can be obtained from the theoretical model (Saito et al., 2010).